An active matrix display device has been proposed in which each pixel has a light emitting element and a transistor for controlling light emission of the light emitting element. A light emitting element refers to an element which has a first electrode and a second electrode and whose luminance is controlled by the amount of current flowing between the first electrode and the second electrode. Display devices using OLED (Organic Light Emitting Diode) elements as light emitting elements (hereinafter referred to as OLED display devices) are attracting attention. OLED display devices have advantages such as excellent responsiveness, low voltage operation, and wide viewing angle, thereby receiving attention as the next-generation flat panel displays.
In active matrix OLED display devices, luminance information is written in each pixel by a voltage signal or by a current signal. The former is called a voltage writing type and the latter is called a current writing type analog method. These driving methods will be described below using examples.
FIG. 30 shows a structural example of a pixel in a conventional voltage writing type OLED display device. In FIG. 30, each pixel has two TFTs (a first TFT and a second TFT), a capacitor element, and an OLED. The first TFT (hereinafter referred to as selecting TFT), which is denoted by 3001, has a gate electrode connected to a gate signal line 3002 and has a source terminal and a drain terminal one of which is connected to a source signal line 3003. The other of the source terminal and drain terminal of the selecting TFT 3001 is connected to a gate electrode of the second TFT (hereinafter referred to as driving TFT), which is denoted by 3004, and to one of electrodes of the capacitor element (hereinafter referred to as storage capacitor), which is denoted by 3007. The other electrode of the storage capacitor 3007 is connected to a power supply line 3005. The driving TFT 3004 has a source terminal and a drain terminal one of which is connected to the power supply line 3005 and the other of which is connected to a first electrode 3006a of the OLED, which is denoted by 3006. A second electrode 3006b of the OLED 3006 receives a constant electric potential. Here, the electrode of the OLED 3006 that is connected to the driving TFT 3004, namely, the first electrode 3006a, is called a pixel electrode whereas the second electrode 3006b is called an opposite electrode.
The description given below is about a driving method for when the selecting TFT 3001 in FIG. 30 is an n-channel TFT, the driving TFT 3004 is a p-channel TFT, the first electrode 3006a and second electrode 3006b of the OLED are an anode and a cathode, respectively, and the electric potential of the second electrode 3006b is set to 0 V.
A signal is inputted to the gate signal line 3002 to turn the selecting TFT 3001 conductive, and then a signal voltage is inputted to the selecting TFT 3001 from the source signal line 3003. Upon input of the signal voltage from the source signal line 3003, electric charges are accumulated in the storage capacitor 3007. In an amount according to the voltage held in the storage capacitor 3007, a current flows into the OLED 3006 through the source-drain of the driving TFT 3004 from the power supply line 3005 and the OLED 3006 emits light.
Voltage writing type display devices having pixels structured as shown in FIG. 30 can employ two types of driving methods, analog method and digital method. Hereinafter, the two are called as a voltage writing type analog method and a voltage writing type digital method.
In the voltage writing type analog driving method, the gate voltage (gate-source voltage) of the driving TFT 3004 in each pixel is changed to change the drain current of the driving TFT 3004. The method thus changes the amount of current flowing in the OLED 3006 to change the luminance. In order to obtain intermediate gray scale, the driving TFT 3004 operates in a range where a change in drain current is large to a change in gate voltage.
The voltage writing type analog method described above has a problem in that the current flowing in the OLED 3006 fluctuates greatly due to changes in drain current caused by fluctuation in current characteristic of the driving TFT 3004 when signals inputted to pixels from their respective source signal lines 3003 have the same electric potential. Fluctuation in current characteristic of the driving TFT 3004 is influenced by parameters such as threshold voltage and carrier mobility. As an example thereof, fluctuation in current characteristic due to fluctuation in threshold voltage of the driving TFT 3004 is described with reference to FIG. 31.
FIG. 31(A) is a diagram showing only the driving TFT 3004 and OLED 3006 of FIG. 30. The source terminal of the driving TFT 3004 is connected to the power supply line 3005. The gate voltage of the driving TFT 3004 is indicated by Vgs in the drawing. The drain current of the driving TFT 3004 is indicated by an arrow Id in the drawing. FIG. 31(B) shows the relation between the absolute value |Vgs| of the gate voltage of the driving TFT 3004 and its drain current Id (the current characteristic). Denoted by 3101a is a curve showing the relation between the gate voltage and the drain current when the absolute value of the threshold voltage of the driving TFT 3004 is Vth1. On the other hand, 3101b is a curve showing the relation between the gate voltage and the drain current when the absolute value of the threshold voltage of the driving TFT is Vth2. Here, Vth1 is larger than Vth2 (Vth1>Vth2). An operation range (1) shown in the drawing corresponds to the operation range of the driving TFT 3004 in the voltage writing type analog method. If the threshold of the driving TFT 3004 fluctuates in the operation range (1), the drain current of one is Id1 whereas the drain current of another is Id2 and the difference is large even though they have the same gate voltage Vgs1. Fluctuation in threshold voltage causes fluctuation in luminance of the OLED 3006 since the luminance of the OLED 3006 is in proportion to the amount of current flowing in the OLED 3006.
The voltage writing type digital driving method has been proposed in order to reduce the above-described influence of fluctuation in current characteristic of the driving TFT 3004. In the voltage writing type digital driving method, the OLED 3006 of each pixel is in a state chosen from light emission at a constant luminance and no-light emission. The driving TFT 3004 in FIG. 30 serves as a switch to select connection between the power supply line 3005 of each pixel and the pixel electrode 3006a of the OLED 3006. While the OLED 3006 is emitting light in the voltage writing type digital method, the driving TFT 3004 operates in a linear range that is an operation range where the absolute value of a source-drain voltage Vds is smaller than the absolute value of Vgs−Vth obtained by subtracting the threshold Vth from the gate voltage Vgs, particularly, in a range where the absolute value of the gate voltage is large.
The operation range of the driving TFT 3004 in the voltage writing type digital method is an operation range (2) in FIG. 31(B). The operation range (2) is a linear range and, in the driving TFT 3004 operating in this range, fluctuation in drain current due to fluctuation in threshold voltage and the like is small and an almost constant current Id3 flows if the same gate voltage Vgs2 is applied. Therefore fluctuation in current flowing in the OLED 3006 is lowered and changes in light emission luminance are reduced.
The relation between the driving TFT 3004 operating in the linear range, the OLED 3006, and voltages applied to 3004 and 3006 is explained with reference to FIG. 32. FIG. 32(A) shows only the driving TFT 3004 and OLED 3006 of FIG. 30 for the explanation. Here, the source terminal of the driving TFT 3004 is connected to the power supply line 3005. The source-drain voltage of the driving TFT 3004 is indicated by Vds. The voltage between the cathode and anode of the OLED 3006 is indicated by VOLED. The current flowing in the OLED 3006 is denoted by IOLED. The current IOLED equals the drain current Id of the driving TFT 3004. The electric potential of the power supply line 3005 is indicated by Vdd. The electric potential of the opposite electrode of the OLED 3006 is set to 0 V In FIG. 32(B), 3202a is a curve showing the relation between VOLED=and IOLED of the OLED 3006 (I-V characteristic). Denoted by 3201 is a curve showing the relation between the source-drain voltage Vds of the driving TFT 3004 and its drain current Id (IOLED) when the gate voltage is Vgs2 in FIG. 31(B). The operation condition (operation point) of the driving TFT 3004 and OLED 3006 is determined by the intersection point of the two curves. The operation point is an intersection point 3203a of the curve 3201 and the curve 3202a in the linear range shown in the drawing since the driving TFT 3004 operates in the linear range. This means that the voltage between the anode and cathode of the OLED 3006 is VA1 and the current thereof is IOLED1.
On the other hand, in display devices having current writing type analog method pixels, a signal current is inputted to each pixel from a signal line (source signal line). Here, a signal current is current signals linearly corresponding to luminance information of video signals. The gate voltage of a TFT whose drain current is the inputted signal current is held in a capacitor portion. In this way, the OLED keeps receiving the current held by the capacitor portion after the source signal line stops inputting a signal current to the pixel. By changing a signal current inputted to a source signal line as this, the amount of current flowing in an OLED is changed to control the light emission luminance of the OLED and display in gray scales.
As an example of the current writing type analog method pixel, FIG. 33 shows a pixel structure disclosed in “IDW '00 p235: Active Matrix PolyLED Displays”, and a driving method thereof will be described. In FIG. 33, a pixel is composed of an OLED 3306, a selecting TFT 3301, a driving TFT 3303, a capacitor element (storage capacitor) 3305, a holding TFT 3302, a light emission TFT 3304, a source signal line 3307, a first gate signal line 3308, a second gate signal line 3309, a third gate signal line 3310, and a power supply line 3311.
A gate electrode of the selecting TFT 3301 is connected to the first gate signal line 3308. The selecting TFT 3301 has a source terminal and a drain terminal one of which is connected to the source signal line 3307 and the other of which is connected to a source terminal or drain terminal of the driving TFT 3303, to a source terminal or drain terminal of the holding TFT 3302, and to a source terminal or drain terminal of the light emission TFT 3304. Of the source terminal and drain terminal of the holding TFT 3302, one that is not connected to the selecting TFT 3301 is connected to one of electrodes of the storage capacitor 3305 and to a gate electrode of the driving TFT 3303. The side of the storage capacitor 3005 that is not connected to the holding TFT 3002 is connected to the power supply line 3311. A gate electrode of the holding TFT 3302 is connected to the second gate signal line 3309. Of the source terminal and drain terminal of the driving TFT 3303, one that is not connected to the selecting TFT 3301 is connected to the power supply line 3311. Of the source terminal and drain terminal of the light emission TFT 3304, one that is not connected to the selecting TFT 3301 is connected to one electrode 3306a of the OLED 3306. A gate electrode of the light emission TFT 3304 is connected to the third gate signal line 3310. The other electrode 3306b of the OLED 3306 is kept at a constant electric potential. Of the two electrodes 3306a and 3306b of the OLED 3306, one that is connected to the light emission TFT 3304, i.e., the electrode 3306a is called a pixel electrode and the other electrode, i.e., the electrode 3306b is called an opposite electrode.
In the pixel structured as shown in FIG. 33, the current value of a signal current inputted to the source signal line is controlled by a video signal input current supply 3312. In practice, plural video signal input current supplies 3312 respectively associated with plural pixel columns correspond to a part of a source signal line driving circuit. In the example shown here, the pixel has n-channel TFTs for the selecting TFT 3301, the holding TFT 3302, and the light emission TFT 3304, and has a p-channel TFT for the driving TFT 3303, and the pixel electrode 3306a serves as an anode.
A driving method of the pixel having the structure of FIG. 33 is described with reference to FIGS. 34 and 35. In FIG. 34, the selecting TFT 3301, the holding TFT 3302, and the light emission TFT 3304 are shown as switches to make it easy to see whether they are in a conductive state or nonconductive state. Pixel states (A1) to (A4) correspond to states in periods TA1 to TA4 in a timing chart of FIG. 35, respectively.
In FIG. 35, G_1, G_2, and G_3 represent electric potentials of the first gate signal line 3308, second gate signal line 3309, and third gate signal line 3310, respectively. |Vgs| is the absolute value of the gate voltage (gate-source voltage) of the driving TFT 3303. IOLED is the current flowing in the OLED 3306. IVideo is the current value determined by the video signal input current supply 3312.
In the period TA1, a signal inputted to the first gate signal line 3308 turns the selecting TFT 3301 conductive and a signal inputted to the second gate signal line 3309 turns the holding TFT 3302 conductive. Then the power supply line 3311 is connected to the source signal line 3307 through the driving TFT 3303 and the selecting TFT 3301. The current amount IVideo determined by the video signal input current supply 3312 flows in the source signal line 3307 and, therefore, when enough time has elapsed to reach the steady state, the drain current of the driving TFT 3303 becomes IVideo and a gate voltage according to the drain current IVideo is held in the storage capacitor 3005. At this point, the light emission TFT 3304 is in a nonconductive state. After the voltage is held in the storage capacitor 3005 and the drain current of the driving TFT 3303 is fixed to IVideo, the signal of the second gate signal line 3309 is changed in the period TA2 to turn the holding TFT 3302 nonconductive.
Next, in the period TA3, the signal of the first gate signal line 3308 is changed to turn the selecting TFT 3301 nonconductive. In the period TA4, a signal inputted to the third gate signal line 3310 turns the light emission TFT 3304 conductive and then the signal current IVideo is inputted to the OLED 3306 through the source-drain of the driving TFT 3303 from the power supply line 3311. In this way, the OLED 3306 emits light at a luminance according to the signal current IVideo.
A series of operations in the periods TA1 through TA4 is called a signal current IVideo writing operation. In the operation, the signal current IVideo is changed in an analog fashion to change the luminance of the OLED 3306 and display in gray scales.
In the timing chart of FIG. 35, the absolute value |Vgs| of the gate voltage of the driving TFT 3303 is increased with time in the period TA1 and an operation of holding a gate voltage according to the drain current IVideo is shown. This corresponds to the case where electric charges are not held in the storage capacitor 3305 when the writing operation is started, or the case where the absolute value |Vgs| of the gate voltage of the driving TFT 3303 that is held in the preceding writing operation is smaller than the absolute value |Vgs| of the gate voltage of the driving TFT 3303 of when a given drain current that is determined by the video signal input current supply 3312 flows in the subsequent writing operation.
If the absolute value |Vgs| of the gate voltage of the driving TFT 3303 that is held in the preceding writing operation is larger than the absolute value |Vgs| of the gate voltage of the driving TFT 3303 of when a given drain current that is determined by the video signal input current supply 3312 flows in the subsequent writing operation, the absolute value |Vgs| of the gate voltage of the driving TFT 3303 is reduced with time in the period TA1 and an operation of holding a gate voltage according to the drain current IVideo is observed.
In the current writing type analog method display device described above, the driving TFT 3303 operates in a saturation region. The drain current of the driving TFT 3303 is determined by a signal current inputted from the source signal line 3307. This means that the gate voltage of the driving TFT 3303 is automatically changed so that a constant drain current flows irrespective of fluctuation in threshold voltage, mobility, or the like.
A pixel structure disclosed in JP 2001-147659 A is shown in FIG. 29 as another example of the current writing type analog method pixel, and a driving method thereof will be described in detail. In FIG. 29, a pixel is composed of an OLED 2906, a selecting TFT 2901, a driving TFT 2903, a current TFT 2904, a capacitor element (storage capacitor) 2905, a holding TFT 2902, a source signal line 2907, a first gate signal line 2908, a second gate signal line 2909, and a power supply line 2911.
A gate electrode of the selecting TFT 2901 is connected to the first gate signal line 2908. The selecting TFT 2901 has a source terminal and a drain terminal one of which is connected to the source signal line 2907 and the other of which is connected to a source terminal or drain terminal of the current TFT 2904 and to a source terminal or drain terminal of the holding TFT 2902. Of the source terminal and drain terminal of the current TFT 2904, one that is not connected to the selecting TFT 2901 is connected to the power supply line 2911. Of the source terminal and drain terminal of the holding TFT 2902, one that is not connected to the selecting TFT 2901 is connected to one of electrodes of the storage capacitor 2905 and to a gate electrode of the driving TFT 2903. The other side of the storage capacitor 2905 is connected to the power supply line 2911. A gate electrode of the holding TFT 2902 is connected to the second gate signal line 2909. The driving TFT 2903 has a source terminal and a drain terminal one of which is connected to the power supply line 2911 and the other of which is connected to one electrode 2906a of the OLED 2906. The other electrode 2906b of the OLED 2906 is kept at a constant electric potential. The electrode 2906a of the OLED 2906 that is connected to the driving TFT 2903 is called a pixel electrode and the other electrode, 2906b, is called an opposite electrode.
In the pixel structured as shown in FIG. 29, the current value of a signal current inputted to the source signal line 2907 is controlled by a video signal input current supply 2912. In practice, plural video signal input current supplies 2912 respectively associated with plural pixel columns correspond to a part of a source signal line driving circuit.
In the example shown in FIG. 29, the pixel has n-channel TFTs for the selecting TFT 2901 and the holding TFT 2902 and p-channel TFTs for the driving TFT 2903 and the current TFT 2904, and the pixel electrode 2906a serves as an anode. Here, consider the current characteristic of the driving TFT 2903 as identical with the current characteristic of the current TFT 2904 for simplification. A driving method of the pixel having the structure of FIG. 29 is described with reference to FIGS. 28 and 27. In FIG. 28, the selecting TFT 2901 and the holding TFT 2902 are shown as switches to make it easy to see whether they are in a conductive state or nonconductive state. Pixel states (TA1) to (TA3) correspond to states in periods TA1 to TA3 in a timing chart of FIG. 27, respectively.
In FIG. 27, G_1 and G_2 represent electric potentials of the first gate signal line 2908 and second gate signal line 2909, respectively. |Vgs| is the absolute value of the gate voltage (gate-source voltage) of the driving TFT 2903. IOLED is the current flowing in the OLED 2906. IVideo is the current value determined by the video signal input current supply 2912.
In the period TA1, a signal inputted to the first gate signal line 2908 turns the selecting TFT 2901 conductive and a signal inputted to the second gate signal line 2909 turns the holding TFT 2902 conductive. Then the power supply line 2911 is connected to the source signal line 2907 through the current TFT 2904, the holding TFT 2902, and the selecting TFT 2901. The current amount Ivideo determined by the video signal input current supply 2912 flows in the source signal line 2907 and, therefore, when enough time has elapsed to reach the steady state, the drain current of the current TFT 2904 becomes IVideo and a gate voltage corresponding to the drain current IVideo is held in the storage capacitor 2905.
After the voltage is held in the storage capacitor 2905 and the drain current of the current TFT 2904 is fixed to IVideo, the signal of the second gate signal line 2909 is changed in the period TA2 to turn the holding TFT 2902 nonconductive. At this point, the drain current IVideo flows in the driving TFT 2903. In this way, the signal current IVideo is inputted to the OLED 2906 through the driving TFT 2903 from the power supply line 2911. The OLED 2906 emits light at a luminance according to the signal current IVideo.
Next, in the period TA3, the signal of the first gate signal line 2908 is changed to turn the selecting TFT 2901 nonconductive. The signal current IVideo is kept supplied to the OLED 2906 through the driving TFT 2903 from the power supply line 2911 after the selecting TFT 2901 is made nonconductive and the OLED 2906 continues emitting light.
A series of operations in the periods TA1 through TA3 is called a signal current IVideo writing operation. In the operation, the signal current IVideo is changed in an analog fashion to change the luminance of the OLED 2906 and display in gray scales.
In the current writing type analog method display device described above, the driving TFT 2903 operates in a saturation region. The drain current of the driving TFT 2903 is determined by a signal current inputted from the source signal line 2907. This means that the gate voltage of the driving TFT 2903 is automatically changed so that a constant drain current flows irrespective of fluctuation in threshold voltage, mobility, or the like if the driving TFT 2903 and the current TFT 2904 in the same pixel have the same current characteristic.
The relation between the voltage applied to an OLED and the amount of current flowing therein (the I-V characteristic) is changed by environment temperature of the surroundings, degradation of the OLED, and the like. Therefore a problem of conventional display devices in which driving TFTs operate in a linear range, typically, voltage writing type digital method display devices, is that the amount of current actually flows is varied even when a constant voltage is applied between two electrodes of an OLED.
FIG. 36 show a change in operation point when the I-V characteristic of an OLED is changed by degradation or the like in a display device using a conventional voltage writing type digital driving method.
FIG. 36(A) is a diagram showing only the driving TFT 3004 and OLED 3006 of FIG. 30. Here, the source terminal of the driving TFT 3004 is connected to the power supply line 3005. The source-drain voltage of the driving TFT 3004 is indicated by Vds. The voltage between the cathode and anode of the OLED 3006 is indicated by VOLED and the current thereof is denoted by IOLED. The current IOLED equals the drain current Id of the driving TFT 3004. The electric potential of the power supply line 3005 is indicated by Vdd. The electric potential of the opposite electrode of the OLED 3006 is set to 0 V.
In FIG. 36(B), a curve 3202a shows the I-V characteristic of the OLED 3006 before degradation and a curve 3202b shows its I-V characteristic after degradation. The operation condition of the driving TFT 3004 and OLED 3006 before degradation is determined by an intersection point 3203a between the curve 3202a and a curve 3201. The operation condition of the driving TFT 3004 and OLED 3006 after degradation is determined by an intersection point 3203b between the curve 3202b and the curve 3201.
In a pixel for which a light emission state is chosen, a gate electric potential that turns the driving TFT 3004 conductive is inputted to 3004. At this point, the voltage between the two electrodes of the OLED 3006 is VA1. When the OLED 3006 is degraded to change its I-V characteristic, the operation point is changed even though the same gate voltage is inputted, and the current flowing therein is changed from IOLED1 to IOLED2 even though almost the same voltage VA1 is applied between the two electrodes of the OLED 3006. The light emission luminance of the OLED 3006 is thus changed according to the degree of degradation of the OLED 3006 in each pixel.
On the other hand, in display devices which have the pixel structure shown n FIG. 33 or FIG. 29 and which use the conventional current writing type analog riving method, the luminance is expressed by a constant current flowing into an OLED. Degradation or the like causes a change in I-V characteristic of the OLED in this case and influence of the change is described with reference to FIG. 37. Components common to FIG. 37 and FIG. 33 are denoted by the same symbols and explanations thereof are omitted. In FIG. 33, the light emission TFT 3304 is simply deemed as a switch and the source-drain voltage thereof is ignored.
FIG. 37(A) shows only the driving TFT 3303 and OLED 3306 of FIG. 33. Here, the source terminal of the driving TFT 3303 is connected to the power supply line 3305. The source-drain voltage of the driving TFT 3303 is indicated by Vds. The voltage between the cathode and anode of the OLED 3306 is indicated by VOLED. The current flowing in the OLED 3306 is denoted by IOLED. The current IOLED equals the drain current Id of the driving TFT 3303. The electric potential of the power supply line 3305 is indicated by Vdd. The electric potential of the opposite electrode of the OLED 3306 is set to 0 V.
In FIG. 37(B), 3701 is a curve showing the relation between the source-drain voltage of the driving TFT 3303 and its drain current. Denoted by 3702a is a curve showing the I-V characteristic of the OLED 3306 before degradation and 3702b is a curve showing the I-V characteristic of the OLED 3306 after degradation. The operation condition of the driving TFT 3303 and OLED 3306 before degradation is determined by an intersection point 3203a between the curve 3702a and a curve 3701. The operation condition of the driving TFT 3303 and OLED 3306 after degradation is determined by an intersection point 3703b between the curve 3702b and the curve 3701.
In the current writing type analog method pixel, the driving TFT 3303 operates in a saturation region. Through degradation of the OLED 3306, the voltage between the two electrodes of the OLED 3306 changes from VB1 to VB2, but the current flowing in the OLED 3306 is kept almost constant at IOLED1. This change in operation condition of the driving TFT and OLED due to a change in I-V characteristic of the OLED applies to the driving TFT 2903 and the OLED 2906 in the pixel structure shown in FIG. 29.
However, the current writing type analog driving method needs to hold electric charges according to a signal current anew in a capacitor portion (storage capacitor) of each pixel each time pixels are used for display. Holding a given level of electric charges in a storage capacitor, when signals are written in a pixel, takes longer as the signal current becomes smaller because of cross capacitance of wirings or the like. Therefore it is difficult to write a signal current quickly.
A small signal current also increases influence of noises such as leak current caused by plural pixels that are connected to the same source signal line other than the pixel in which a signal current is being written. Accordingly there is a strong possibility that the pixel cannot emit light at an accurate luminance.
In a pixel structure having a current mirror circuit, which is represented by a pixel as the one shown in FIG. 29, a pair of TFTs whose gate electrodes are connected have to have the same current characteristic in the current mirror circuit. However, in practice, matching the current characteristics of the TFTs that forms a pair exactly is difficult and it results in fluctuation.
Here, the driving TFT 2903 and current TFT 2904 of FIG. 29 are given a threshold Vtha and a threshold Vthb, respectively. Now let us examine displaying black when their threshold fluctuates and the absolute value |Vtha| of Vtha is smaller than the absolute value |Vthb| of Vthb. The drain current lowing in the current TFT 2903 corresponds to the current value IVideo determined by the video signal input current supply 2912 and is zero. However, there is a possibility that a voltage slightly smaller than |Vthb| is held in the storage capacitor 2905 although no drain current flows in the current TFT 2903. Then the drain current of the driving TFT 2903 might not be zero since |Vthb|>|Vtha|. In this way, a drain current flows in the driving TFT 2903 to cause the OLED 2906 to emit light even though black display is intended. This brings a problem of reduction in contrast.
Furthermore, conventional current writing type analog method display devices have a video signal input current supply for inputting a signal current in each pixel for each column of pixels, and the devices have to make current characteristics of all the video signal input current supplies match and control the current value so as to change the current value accurately in an analog fashion. For that reason, it is difficult to manufacture video signal input current supplies having the same current characteristic in transistors that use a polycrystalline semiconductor thin film. The video signal input current supplies are therefore manufactured from an IC chip. On the other hand, pixels are generally formed on a glass or other insulating substrate (a substrate having an insulating surface) from cost and other reasons. Then the IC chip has to be bonded to the glass or other insulating substrate. Bonding the chip requires a large area, which is a problem because it makes reduction of the frame area in the periphery of the pixel region impossible.
The present invention is proposed in view of the above, and has an object of providing a low-cost display device with a reduced size in which a light emitting element can emit light at a constant luminance irrespective of a change in current characteristic due to degradation or the like, which is fast in writing signals in pixels, and which is capable of displaying in precise gray scales, as well as a method of driving the display device.